ON THE FOETAL MEMBRANES OF CHELOXIA. o!^ 



time before the meso})lastic folds insinuate theniselves into the epi- 

 blastic sheel. I think, hijvvcver, thai ahliDUg'h these folds are solid 

 and \vith<.)ut any cavity, they ought t(.) he regarded as consisting of t\V(j 

 Jinibs, the inner and outer, which are hrinl\' ap|)ressed against each 

 otlier : otlierwise there is no reason \N'hy the .sero-anini(.)tic connection, 

 which ought to be regarded as the seam al«jng which the folds of the 

 two sides have met. should remain permanently and sej)arate the 

 mesoblastic idlds of the two sides to the end. 



If the doi'sal part of tiie proanuiic.n consisted primarily of t he 

 e[)ibhist ahjne, why sliouM the meso]>lastic folds afterward insinuate 

 tliemsehes between tlie two limbs of that ])art, thus extending tlie 

 extra-eml)ryonic c<r']omic ca\it\ into that region? For the explana- 

 ti(jn ofthat process, I adopt Jxdfour's view. To give efücac\' to the 

 aliantois as a respiratory organ, it is desira])le that it should l)e 

 spread as extensixely as possibk^ chjse under the surface of the egij!; ; 

 hence the extra-em Ijrycjnic «-adomic cavity must ha\e spread pai'i- 

 jHissii willi the gradual growth of the aliantois. The extension of 

 tile folds of the somatic mesobiasf into the epiblastic fdds of the 

 [)roaninion is. I think, due ])rimai-i]y tn this caiise. That the 

 mesohJast s))reads itself at present long Ijetore the aliantois, is to be 

 ex])lained as a case of precocious dc^eh^j^nent. 



rile sero-amiiioidc coiiiiection is, in m\' oitiiiion, decidedly a 

 pi'imitixe structure The manner in which t he aliantois sjU'eads itself 

 in (Memniys hy r»junding the sero-amniotic connection can also 

 be explaine(l only on ])hylogeiiet ic grounds. The manner in which 

 the allantoic blood-vessels -are slung <jn mesenter\ -like folds of the 

 aliantois, is, i think, also a primitive feature. The manner in 

 which the sero-amniotic c<jnnection in Trionyx is pushed forward, 

 bent and compressed, points out, I think, the way in which that 

 structure historically disappeared in higher forms. As i have stated 



